Liftgates and hoods are hingedly mounted to vehicles and pivot between an opened and closed position to grant access to a cargo area or an engine compartment respectively. Gas struts are used to assist opening the liftgate or hood and support the same in the open position. Referring to the liftgate, a pair of struts, one on each side of the liftgate, has an end connected to the liftgate and another end connected to the vehicle body. Both struts are identical with each strut having a cylinder-and-piston arrangement with a chamber within the cylinder defined by a first end of the cylinder and the piston, which is filled with a pressurized gas. The pressurized gas end pushes against the piston urging the strut to an extended position in which the liftgate is opened. The strut is held in a retracted position by the liftgate being latched to the vehicle resisting the bias of the pressurized gas. These struts are commonly called pneumatic springs or gas springs.
It is known that the force exerted by the strut is a function of the gas pressure. It is also known that the gas pressure in the cylinder is affected by the temperature, since the pressure increases as the temperature increases if the volume is constant. It is necessary that it be possible to close the liftgate of the vehicle against the bias of the strut. Therefore, the amount of gas placed in the chamber during manufacturing must take into account the highest temperature the strut is likely to encounter, since the pressure of the gas is highest at this highest temperature resulting in highest force exerted. During cold weather, the strut will exert the minimum amount of force which could result in the liftgate not being held in the fully opened position or the force exerted could be too weak to maintain the liftgate opened at all. There has been proposed previously various methods to secure the liftgate in the opened position by mechanically locking the piston relative to the cylinder.
It would be desirable to have a non-mechanical means for securing the strut in the extended position compensating for variation of force of strut caused by temperature.